[Petrus]

PMI = polar moment of inertia.

Simply put mass times the distance to centre of giration squared.
This explains why a mid engined car changes direction easier than front or rear engined-

It also explains why the effect of lighter wheels is both diproportionately large and poorly understood.
Same thing the appearantly too large effect on stabilty/understeer of the spare wheel in/out of the front bucket whereas a full versus empty tank does not affect it.

Today deleted the front crash bar and the OEM washer fluid bottle*.
When filled up, the bottle and bar add up to 10 kg.
They sit some 2 m from the CM.

Should have more effect than simply 10 kg of lightness added. More so because my car already was sub 900 kg.

* will fit a small capacity plastic bag with pump weighing 150 gramms empty.

 

[03MRS]

how did you get spyder to sub 900 kg? thats about 100kg (200lbs) off stock weight?

 

[rmeller]

I doubt that you will notice a difference in the turn-in time of the car, considering that the total yaw inertia is something like 1000 kg m^2. This is a real effect, but I cannot think of a way to objectively measure a performance difference. Of course, everything adds up, and if you keep moving stuff, maybe you can get to a significant change.

Same with the wheels, although they do affect acceleration, which is measurable. The component that really makes a difference is the flywheel. That is because the effective inertia of the flywheel is multiplied by the square of the total gear ratio, and so in 1st gear a kg at the periphery of the flywheel could be equivalent to 100kg or more of ballast mass.

 

[Petrus]

how did you get spyder to sub 900 kg? thats about 100kg (200lbs) off stock weight?

100??
Was 1060 kilos. Had it weighed at 900. Since then added heavier swaybars, took some more off, should be 890. Lóóóng story.

Would swap bonnet and rear lid for lightweight too if that was affordable over here. Same thing lexan windscreen.
A moderately lighter flywheel is on the list too if/when the clutch needs attention.

 

[Petrus]

I doubt that you will notice a difference in the turn-in time of the car, considering that the total yaw inertia is something like 1000 kg m^2. This is a real effect, but I cannot think of a way to objectively measure a performance difference. Of course, everything adds up, and if you keep moving stuff, maybe you can get to a significant change.

Same with the wheels, although they do affect acceleration, which is measurable. The component that really makes a difference is the flywheel. That is because the effective inertia of the flywheel is multiplied by the square of the total gear ratio, and so in 1st gear a kg at the periphery of the flywheel could be equivalent to 100kg or more of ballast mass.

Stopping distance is a hard one too and do not forget cornering forces.

Subjectives are straight line stability, oversteer/understeer, response to traction loss/recovery, tramlining, to name but a few.

Yes, the effects of the reduction in wheel mass are difficult to méasure without übercool tools on a test rig.
This is the root cause for the wildy differing claims about what 1 kilo of added lightness there does.
The méasurable difference is most marked in stopping distance. In féél however the effects are larger. Mý subjective perception is that because the gyroscopic and polar moment effects are so much less, the feed back by the forces from the róad become relatively more marked, the differences you féél get larger.
Lighter wheels on the MR2 have a relatively larger effect on the PMI as the wheelbase is long and the wheels sit far out from the COG.

As to the relative effect of the PMI reduction you are correct.
It is however like taking off 1 kg. off an 1060 kg car. Makes pretty much no sense on its own. Now repeat that all over the car and it adds up to 160 kg i.e. 15% off! You sure notoce thát! and as you take off kilos, the effect of each kilo gets progressively larger.
Back to PMI that 160 kg lighter car has subsequently less rotational inertia and taking off 10 kilos at the far ends has a relatively larger effect.

Btw should your 1000 kgm not be less in thís case of a mid engined car as the average distance of the weight is not 1 m.?
The PMI is the resultant from the difference between center of gravity and axis of the rotation. Take the 1060 and a distance of 30 cm and you get 858 kgm. I remember having seen a value for the Mk1 and that was less than 30 cm. whereas the Mk3 should have an even better number.
Aaaaaanyway; regardless of 1000, 800 or whatever, the point you make is valid; the total PMI is wáy larger than that of the 10 kg at the far ends so the effect not changing the car.
As is my point that all effects added up dó

Oh, the 500 gramm (filled) washer fluid bag works!! That is a lót of kilos for beer money and as a bonus the squirt has more force.

 

[03MRS]

well, if you ever get time, Please let us know. I am very interested in details what you have taken out to achieve such weight loss, and I am sure others would too. Also, was 1060kg weight with fuel or why was it so high initially?

 

[Petrus]

well, if you ever get time, Please let us know. I am very interested in details what you have taken out to achieve such weight loss, and I am sure others would too. Also, was 1060kg weight with fuel or why was it so high initially?

With fuel, airco, smt, lsd.

I have a thread with the work in progress on MR2 Roadster Owners Club - Index
Most should be in it.
Biggest gains were exhaust (header, decat, superbike ´silencer´), bucket seats, airbags, p.a.s., airco, sound sytem (incl. speakers) and Enkei RPF1s.
Last things are those mentioned above.

 

[03MRS]

With fuel, airco, smt, lsd.

I have a thread with the work in progress on MR2 Roadster Owners Club - Index
Most should be in it.
Biggest gains were exhaust (header, decat, superbike ´silencer´), bucket seats, airbags, p.a.s., airco, sound sytem (incl. speakers) and Enkei RPF1s.
Last things are those mentioned above.

Can't find the thread, link pls? I am very concerned about all these, since getting k24 swap I am basically adding 100lbs (yes granted in front of rear axles but still rear bias)

 

[phattires]

Don't worry you wont notice the extra weight ....

here I looked it up for you ....

Southern Belle

Southern Belle

www.mr2roc.org

 

[silversprint]

What are you concerned about? 100lbs of weight is not going too be of much significance. That k24 is probably adding 90hp in power for just 100lbs added.

To gain similar power to weight with a stock 1zz you would need to reduce the spyder weight to 1400lbs. Even if you completely strip a spyder and just keep a driving chassis it's still heavier than 1600lbs.

The corner weights on the rear of a spyder is about 700lbs. So now you have 750lbs. Big deal. Thats easily managed with proper suspension.

Your k24 is going to spank any 1zz spyder regardless of weight.

Enjoy the car.

 

[Funkycheeze]

Yeah definitely don't start removing stuff like airbags and crash bars to reduce weight LOL.

 

[03MRS]

What are you concerned about? 100lbs of weight is not going too be of much significance. That k24 is probably adding 90hp in power for just 100lbs added.

To gain similar power to weight with a stock 1zz you would need to reduce the spyder weight to 1400lbs. Even if you completely strip a spyder and just keep a driving chassis it's still heavier than 1600lbs.

The corner weights on the rear of a spyder is about 700lbs. So now you have 750lbs. Big deal. Thats easily managed with proper suspension.

Your k24 is going to spank any 1zz spyder regardless of weight.

Enjoy the car.

You are probably right, I am just too excited to try it out, and then be disappointed (doubt I will be, just scared---should have it in few weeks). More weight should also hopefully be balanced out by LSD. ------ More weight in the back should provide more grip,--->understeer, however I am worried about more Snap Oversteer, hopefully that can be corrected by stiffer setting on sway bar? Am I correct in this? (I am much better in human mechanics not car)

 

[FormInFunction]

First off....you won’t notice the weight. Whatsoever. It’s not being mean, you don’t have the skill & experience required for it to make an impact. Those that do don’t worry about it, they adjust themselves, the car, or both.

Secondly, if you were really that worried about weight you would have corner balanced the car, both before & after the build, then considered things such as battery relocation to bring that % balance back to where it was. ..all of which fairly unnecessary for that weight difference on a street car. If you’re that nervous about your abilities regarding a 100 pound difference there is no way I’d ever get in your passenger seat.

Your problem is you’re thinking theoretically about problems you don’t have the experience to know if you will encounter. A passenger is way more of a difference to the car than 100lbs at the engine block, not to mention that some of the added engine weight should be offset by lighter exhaust components, better tuned suspension, etc if you really care that much. When you finally drive the car you’ll settle into it just fine. At some point you’ll spin and blame the weight differences, when it was the driver all along. We’ve all been there, and anyone that pushes to the point that a 5% weight change makes a difference in performance will be there again.

 

[03MRS]

I have never upset weight balance on Mid-engined cars before, so that is why I am concerned. average passenger is what? lets say about 150lbs (for argument sake). That weight is added to the center of the vehicle. I can most definitely tell difference as to vehicles ability to rapidly change direction. Weight of the engine is added far more closer (almost on top) of rear "axle". I quote 100 lbs as estimate considering all the weight saving I will actually get from lighter components, especially replacing that huge overweight muffler hanging from far end of the car. I do have experience with BMW gutting and stripping out--- what mostly happens is that you loose weight off back tires--- and have to adjust suspension and driving style for that. (no more "magical" 50/50 weight lol), but that set-up is more predictable to me.

Now, I will have gutted front, (no spare, no bin, plastics, tools, Power steering) + added weight on back. This combo will definitely need some adjustment.

On a side note: --- I have never spun this car with a passenger in it, however have when trying to see cars limit, and it is pretty hard to correct, as there is almost no warning.

Oh and trust me, ALOT of people are afraid to get in this car lol (even back when it was stock 1zz)

 

[Petrus]

I have never upset weight balance on Mid-engined cars before, so that is why I am concerned. average passenger is what? lets say about 150lbs (for argument sake). That weight is added to the center of the vehicle. I can most definitely tell difference as to vehicles ability to rapidly change direction. Weight of the engine is added far more closer (almost on top) of rear "axle". I quote 100 lbs as estimate considering all the weight saving I will actually get from lighter components, especially replacing that huge overweight muffler hanging from far end of the car. I do have experience with BMW gutting and stripping out--- what mostly happens is that you loose weight off back tires--- and have to adjust suspension and driving style for that. (no more "magical" 50/50 weight lol), but that set-up is more predictable to me.

Now, I will have gutted front, (no spare, no bin, plastics, tools, Power steering) + added weight on back. This combo will definitely need some adjustment.

On a side note: --- I have never spun this car with a passenger in it, however have when trying to see cars limit, and it is pretty hard to correct, as there is almost no warning.

Oh and trust me, ALOT of people are afraid to get in this car lol (even back when it was stock 1zz)

´You´ will hardly notice a full versus near empty tank yet you will notice half that weight from the extreme ends. The difference is the PMI wich is weight time distance (from center of movement) squared.

I validate your question. It is not about weigth/power but about the distribution.
In the case of the engine, the weight is very close to the center of movement so will not affect the PMI much. Probably akin to the full/empty tank.
It wíll however affect the front/rear tyre loads a bit. Nothing beyond sway bar adjustment though. You might need to go a hole softer, which will suit the extra power too.

The halowed 50/50 btw is internet spread urban myth originating from front engined rear wheel drive and totally wrong for most cars and définitely wrong for mid engined cars. For the MR2 Spyder you want some rear weight bias and depending on your use most likely more rather than less than standard.

Wishing you búcket loads of fun with your car!!

 

[Petrus]

First off....you won’t notice the weight. Whatsoever. It’s not being mean, you don’t have the skill & experience required for it to make an impact. Those that do don’t worry about it, they adjust themselves, the car, or both.
....
If you’re that nervous about your abilities regarding a 100 pound difference there is no way I’d ever get in your passenger seat.

Why these subjective judgemental opinions again?!

I could counter this with venting that if you do not notice 150 lbs weight more/less then you .... etcetera. That does not add anything helpful though.

Have a look on youtube at a Japanese ´Touge battle´ comparison between the 6 pot Supra and the new 4 pot one. Should enlighten some here about 75-100 kilos and minor PMI change on a 1400-1500 kilo car.

 

[FormInFunction]

Why these subjective judgemental opinions again?!

I could counter this with venting that if you do not notice 150 lbs weight more/less then you .... etcetera. That does not add anything helpful though.

Have a look on youtube at a Japanese ´Touge battle´ comparison between the 6 pot Supra and the new 4 pot one. Should enlighten some here about 75-100 kilos and minor PMI change on a 1400-1500 kilo car.

This is not subjective, nor is it an opinion. It is objective, it is a fact. You are referencing professional drivers that a single change to the car will impact their time. Soft or hard compound tires will impact their time.

I realize that you have this need to argue, to feel that you are the victim but this is not about you personally whatsoever. If someone else can consistently place a significantly better time, then the original driver in question is not skilled enough for such things to matter. Period.

 

[Petrus]

Oh my, you are an expert psychologist as well. Impressive.

As to single factors; your perception, your reality and you are welcome to it. You not noticing the weight of a passenger puts it in perspective

I will stick to moddifying óne thing at a time and test ceterus paribus both the objective and subjective.

 

[FormInFunction]

I read that twice and at this point I honestly have no clue what you are:
1. Trying to say
2. Actually saying
3. Think you said

Psychology had not previously been brought into this discussion. I’m not sure where the misunderstanding lies.

 

[Mr2Yaj]

Removing weight at the far end of the car is more significant than at the center of the car, just as you explained. Although this weight saving is something I'll prob never do unless it becomes a track car, it's an interesting topic. It's also something ppl overlook. 100lbs, depending where it is and how experienced of a driver you are makes a shit load of a difference to the car. Especially at a track where your pushing the car to the limit. You prob won't notice it driving around town. A 100lbs passenger is not the same as 100lbs in the frunk, in front of the axles, or behind the engine. This is exactly why mid-engined cars are superior to front engine cars. The weight is placed more in the middle instead of at the far front of the car where it extends beyond the front struts or in rear engine cars where it sits behind the rear axles. This is why cars like the s2000, cayman, mr2 spyders, and other mid engine cars excels on the track. Yes 50/50 weight distribution is a myth. It's a marketing scheme. You can have 1000 pound at the front end of the car and 1000 pounds at the rear end of the car and it'll handle like shit and still be the praised 50/50 weight distribution. Rear bias weight distribution is superior especially if that weight is near the
middle of the car.